Reuse or recycling of old asphalt pavement is much in vogue these days. The old pavement is first removed and sized to produce an aged mix or "RAP", as it is often called. To produce a recycled mix the RAP is then combined with fresh or virgin aggregate, "VAM" as it too is often called, and new liquid asphalt, all typically in apparatus of the drum mixer nature. One of the problems inherent in this type of apparatus is to prevent the hot flame of the burner from firing or coking the old asphalt in the RAP before the latter joins the VAM and the fresh asphalt. Not only does coking of the old asphalt damage it but also produces excessive clouds of odious smoke forbidden by environmental regulations. Consequently, the almost universal practice nowadays is to introduce the RAP at a point well downstream of the burner in order to shield the RAP as much as possible.
One technique of performing this is described in U.S. Pat. No. 4,165,184 to Schlarmann in which a cylindrical inner drum extends part way down the main drum through its upstream end, the flame of the burner and the VAM entering the inner drum while the RAP enters the annular space between the two drums. The RAP is thus shielded from direct contact with the flame until it joins the VAM well downstream of the burner. Added advantages are that the RAP is heated by the hot exterior surface of the inner drum before it joins the VAM and the lesser diameter of the inner drum increases the "veil" of VAM across the flame in order further to protect the RAP. The Schlarmann arrangement performs well enough in many instances, but it was later discovered that if the plant is operated at high production rates it is difficult to get sufficient VAM through the inner drum owing to its lesser diameter. At the same time the consequent increased air velocity through the inner drum tends to entrain too many fines from the VAM. Furthermore, it turned out that if the plant is operated with RAP quantities in excess of fifty percent or so of the total, the proportionately lesser quantity of VAM reduced its screening effect or "veil" across the inner drum enough so that the RAP, even though not exposed to direct flame until well downstream of the burner, was nevertheless inadequately protected from coking.
Subsequent to Schlarmann there appeared the earlier of two versions of a recycle drum mixer combining in effect the Schlarmann principle and that, for example, of U.S. Pat. No. 3,999,743 to Mendenhall. In the latter patent the RAP is introduced midway or so down the drum through circumferentially spaced feed ports through the drum wall, the ports being enveloped by a shroud within which scoops attached to the drum revolve. That earlier version of a recycle drum mixer just referred to likewise features similar feed ports and an enveloping shroud intermediate the ends of the drum but also includes a short conical inner drum disposed with its smaller end directed downstream. The RAP entering the feed ports falls upon the larger upstream end of the inner drum which abuts the inner wall of the outer drum. As in Schlarmann the RAP is thereupon heated and falls off the downstream end of the inner drum to join the heated VAM, the RAP being thereby shielded from the burner by the inner drum until that occurs. No flighting is used on either the interior or exterior of the inner drum. Also, as in Schlarmann, the smaller downstream end of the inner drum increases the density of the "veil" of the VAM across the burner flame just upstream of the junction of the VAM and the RAP. However, only one side wall of the shroud concerned is stationary, its remaining side and circumferential walls being fixed to and revolving with the drum. Not only is sealing between the stationary and revolving parts of the shroud consequently more difficult, but the RAP must enter the shroud through its stationary side wall. This is accomplished by a feed chute that opens into the shroud tangentially. Thereupon, a series of angled flights carried by the outer drum and the two revolving walls of the shroud direct the RAP into the feed ports from which it falls onto the inner drum. However, since those flights are fixed to the revolving portions of the shroud there is no cleaning action by them upon the two walls involved which thereby can become caked with RAP. Furthermore, the RAP entering the shroud must make two right angle turns, first from the feed chute through the stationary side wall of the shroud and then from the interior of the latter into the feed ports.
In the later version of the same apparatus, flighting is provided on the interior, but not on the exterior, of the inner drum, the feed chute is moved to atop the stationary shroud side wall, and the angled flights are omitted. Instead, a series of angle blades attached to the revolving circumferential wall of the shroud and aligned with the edges of the feed ports are relied upon to direct the RAP into the ports. While this arrangement eliminates the second right angle turn the RAP must take in the earlier version yet caking of the interior of the shroud and the lack of self-cleaning action are still present. Furthermore, in both versions any RAP which does not immediately fall into the feed ports when it enters the shroud must then be carried around within the shroud during one or more revolutions of the drum which also adds to wear and caking of the shroud. And, since no flighting is used on the exterior of the inner drum, in either version, considerable RAP, instead of moving downstream of the inner drum, slides circumferentially of the latter directly into the bottom of the outer drum, impairing its heating and mixing with the VAM.
Returning now to the Mendenhall patent mentioned above and others of his, see for example, U.S. Pat. No. 4,215,941, the shroud about the feed ports is stationary. The scoops revolve within the shroud, pick up the RAP introduced into the shroud, carry it around the latter, and finally dump it into the feed ports, causing considerable wear as well as caking on the interior of the shroud since the RAP travels nearly the entire circumference of the shroud before entering the ports. A later modification of that approach is shown in U.S. Pat. No. 4,147,436 to Garbelman et al. in which the scoops are replaced with paddle-like devices called "funnels" and the feed ports equipped with hinged covers which prevent material in the bottom of the drum from falling back into the shroud through the ports. This avoids some of the previous wear and caking of the shroud but the covers often stick, are themselves subject to wear from the VAM passing over them as it joins the RAP and besides are relatively cumbersome and expensive. Another variation of the Mendenhall approach also uses a stationary shroud but fits chutes to the ports which angle back along the inner wall of the drum in a trailing direction relative to its rotation. But these chutes too are subject to high wear by the passing VAM which in addition can fall back through the chutes into the shroud. Moreover, in all the arrangements just mentioned the shroud must seal against the hot surface of the drum since the area of the latter concerned is not shielded from the burner flame. Nor is there any heating or shielding of the RAP before it joins the VAM.